Orthogonal Frequency Division Multiple Access (OFDMA) technology is getting very popular in modern communication systems since the OFDMA technology can efficiently support multiple mobile stations with limited bandwidth and easily provide Quality of Service (QoS). The OFDMA technology is a multiple access version of orthogonal frequency-division multiplexing (OFDM). OFDM is a modulation technique for data transmission based on frequency-division multiplexing (FDM), which uses different frequency channels to transmit multiple streams of data. In OFDM systems, a wide channel is divided into multiple narrow-band subcarriers, which allow orthogonal modulated streams of data to be transmitted in parallel on the subcarriers.
In OFDMA systems, multiple subscribers can simultaneously use different subcarriers for signal transmission. Thus, in an OFDMA system, multiple data bursts can be transmitted from a base station to multiple mobile stations in the same time frame but allocated in different frequency subcarriers. Consequently, an OFDMA system can support multiple mobile stations using different subcarriers.
In a conventional OFDMA transmitter, outgoing data is processed for wireless transmission as OFDMA signals, including being forward error correction encoded. Under the mobile WiMAX standard, the outgoing data can be encoded using a double-binary circular recursive systematic convolutional (DBCRSC) encoding scheme. Thus, in an OFDMA receiver, the incoming OFDMA signals must be decoded using an appropriate decoding technique to extract the original data in the signals. One of the techniques to decode the incoming DBCRSC encoded OFDMA signals is Turbo decoding. However, Turbo decoding has been applied in the past to situations where the initial and final states of the encoded signals were known to the receiver. Under the mobile WiMAX standard, the initial and final states are unknown to the receiver. The only information known to the receiver is that the initial and final states of the encoded signals are the same. Thus, conventional Turbo decoding techniques are not appropriate to decode DBCRSC encoded signals under the mobile WiMAX standard.
In view of this requirement, there is a need for a Turbo decoder and method for Turbo decoding a DBCRSC encoded signal in which initial and final states of the encoded signals are unknown.